In the manufacture of paper products, inorganic fillers and pigments have been widely used for filling and coating applications. These fillers provide many benefits to the finished paper product, such as brightness, opacity, printability and dimensional stability, while also reducing manufacturing costs, energy consumption and increasing manufacturing speeds. Despite these benefits the use of inorganic fillers in tissue paper, which is often substantially lower in basis weight and higher in bulk compared to conventional paper, have been limited.
One limitation is the retention of fillers during the tissue manufacturing process, which often involves machine speeds greatly exceeding the speeds of conventional paper machines and involves exceedingly high degrees of shear. Retention of fillers in tissue products is further challenged by low basis weight of the tissue web. To compound the difficulties in retention caused by the low basis weight, tissue webs are generally low density. Those skilled in the art will recognize that such light weight, low density structures do not afford any significant opportunity to filter and retain fillers in the embryonic web. As a result, filler particles readily pass through the web and are expelled from the embryonic web as it is dewatered.
A second limitation is the general failure of particulate fillers to naturally bond to papermaking fibers in the fashion that papermaking fibers tend to bond to each other as the formed web is dried. This reduces the strength of the product. Filler inclusion causes a reduction in strength, which if left uncorrected, severely limits products which are already quite weak.
Finally, a third limitation is that tissue products containing fillers are prone to lint or dust. This is not only because the fillers themselves can be poorly trapped within the web, but also because they have the aforementioned bond inhibiting effect which causes a localized weakening of fiber anchoring into the structure. This tendency can cause operational difficulties in the creped papermaking processes and in subsequent converting operations, because of excessive dust created when the paper is handled. Another consideration is that the users of the tissue products demand that they be relatively free of lint and dust.
Consequently, there remains a need in the art for fillers for use in tissue webs and products that overcome the limitations of inorganic fillers commonly employed in the manufacture of conventional paper products.